All the Time in the World - The Adventures of Mackenzie Mortimer, #3

Time is running out. but fortunately, Mackenzie Mortimer has few more minutes than anyone else. Mackenzie Mortimer is a typical junior high geek: shy, awkward, late with his homework, and always late for class. But when Mac finds his grandfather's pocket watch, he discovers his days have an extra hour. The eccentric inventor's journal reveals the watch can add up to 60 minutes to a single day by freezing time around whomever controls it.  After the events of The Tomorrow Paradox (Book 2 in the series) Mac has left Gramps' diary behind in the future and Victoria Carmichael's ex-husband, Jeremy Bentaine, has stolen it along with Gramps' journals.
Now, Bentaine Industries seeks to be the sole possessor of time travel technology... and that means eliminating Mackenzie Mortimer before he discovers the pocket watch! But Mac has followed Gramps' trail to WWII Nazi-occupied Belgium, where he must use his future technology to stay alive, locate Gramps, and unite a pair of star-crossed lovers. The horrors of WWII that Mac encounters will change him forever, but if the Bentaine family has its way, forever will be a very short time for Mackenzie Mortimer!